1. Technical Field
Embodiments of the present invention relate to a liquid crystal display device and, more specifically, to a liquid crystal display device in which one of an upper electrode layer and a lower electrode layer formed on an identical substrate via an insulating layer is assigned as a common electrode layer, and the other one of the upper electrode and lower electrode is assigned as a pixel electrode layer, the upper electrode layer is formed with slits, through which voltage is applied between the upper electrode layer and the lower electrode layer for driving liquid crystal molecules.
2. Related Art
In the related art, TN (Twisted Nematic) system has been widely used as a display system of the liquid crystal display device. However, this system has a limit in angle of visibility because of the display principle thereof. As a known method for solving this limitation, there is a lateral electric field system configured in such a manner that a pixel electrode and a common electrode are formed on an identical substrate, and voltage is applied between the pixel electrode and the common electrode to generate an electric field which is substantially parallel to the substrate to drive liquid crystal molecules within a plane which extends in parallel to the substrate plane.
IPS (In plane Switching) system and FFS (Fringe Field Switching) system are known as the lateral electric field system. In the IPS system, a comb-teeth-shaped (or comb-tooth-shaped) pixel electrode and a comb-teeth-shaped (or comb-tooth-shaped) common electrode are arranged in combination. In FFS system, one of the upper electrode layer and the lower electrode layer formed via the insulating layer is assigned as the common electrode and the other one of those is assigned as the pixel electrode, and slits or the like are formed on the upper electrode layer as openings for allowing passage of the electric field.
The openings for allowing the passage of the electric field to the upper electrode layer are formed by etching an electrode thin film. However, when forming the openings into an elongated groove shape, the edge portions at the ends of the long side are formed into a round shape or an arcuate shape in many cases due to the limit of the formation process (exposure, etching). The electric field passing from the lower electrode layer through the openings toward the upper electrode layer flows along the pattern of the openings, and hence the electric field is rounded along the pattern of the arcuate shape at the edge portions. Consequently, for example, when the initial alignment of the liquid crystal molecules is oriented substantially parallel to the long sides of the openings through a rubbing process or the like and the electric field is applied between the upper electrode layer and the lower electrode layer to drive the liquid crystal molecules, the liquid crystal molecules in the areas within the linear portions of long sides of the openings rotate from the initial alignment in the direction vertical to the long sides, while the liquid crystal molecules in the edge portions of the openings rotate from the initial alignment in the direction vertical to the arcuate shape.
When the liquid crystal molecules rotate from the initial alignment along the arcuate shape of the edge portions, such phenomenon that the direction of rotation of the liquid crystal molecules is inverted occurs. For example, it is assumed that the arcuate shape of the edge portions is a part of a circle and the rubbing direction is substantially parallel to the long sides of the openings, and that the direction of the long sides of the openings is X-axis, the direction vertical to the long sides is Y-axis. Then, when the arcuate shape at the edge portions has a shape corresponding to the first quadrant of a circle, the liquid crystal molecules rotate counterclockwise from the X-axis direction and are oriented in the direction vertical to the arcuate shape. In contrast, when the arcuate shape of the edge portions has a shape corresponding to the fourth quadrant of the circle, the liquid crystal molecules rotate clockwise from the X-axis direction and are oriented in the direction vertical to the arcuate shape. In the same manner, when the arcuate shape of the edge portions has a shape corresponding to the second quadrant of the circle, the liquid crystal molecules rotate clockwise from the X-axis direction and are oriented in the direction vertical to the arcuate shape, and when the arcuate shape of the edge portions has a shape corresponding to the third quadrant of the circle, the liquid crystal molecules rotate counterclockwise from the X-axis direction and are oriented in the direction vertical to the arcuate shape.
In this manner, when the electric field is applied, the direction of rotation of the liquid crystal molecules at the edge portions varies depending on the position. The phenomenon that the direction of rotation varies with the position is referred to as “disclination”. At the boundaries between portions demonstrating different directions of rotation, the liquid crystal molecules rotate in undesired directions, or even cannot rotate. Therefore, the coefficient of transmission is lowered, and hence the boundaries are visually recognized. These boundaries are referred to as “disclination lines” or “disclination defects”, and may also be referred simply as “disclination”.
JP-A-2003-280017 points out a problem in the IPS system in which voltage is applied to the comb-teeth-shaped pixel electrodes and the common electrodes formed on the identical substrate to generate the electric field which is substantially parallel to the substrate plane and drive the liquid crystal molecules in a plane parallel to the substrate plane such that radial electric fields generated in the vicinity of distal end portions or root portions of the comb-teeth-shaped electrodes generate areas in which the liquid crystal molecules rotate in the opposite direction from the normal rotation (reverse domain), which generates the disclination defects (disclination) at the boundaries with respect to the areas of normal rotation. Japanese Unexamined Patent Application Publication No. 2003-280017 discloses that when a high-field is generated by superimposing the pixel electrodes and the common electrodes, the normal rotation of the liquid crystal molecules is induced without fixing the same in the direction of the initial alignment by establishing the relation between the direction of a high-field θSE and the direction of initial alignment of the liquid crystal θLC to be θLC<θSE≦θLC+π/2, where the direction of rotation of the liquid crystal is the positive direction (clockwise rotation) with reference to the direction of the scanning wiring (gate wiring), so that occurrence of reverse domain and disclination are restrained.
According to JP-A-2005-107535, in the case of a configuration in which a black matrix on the upper substrate and the edge portions of the pixel electrodes of the lower substrate are overlapped with each other by a predetermined area and liquid crystal is interposed between these substrates, the twisted angle of the liquid crystal molecules is changed gradually to about 90° and arranged in the vertical direction as it approaches from the ends of the edge portions to the center of the pixel electrodes due to the electrical field interference between the black matrix and the pixel electrode. However, since the edge portions have a curved shape due to the limitation in exposure process, a trace of rubbing, that is, the disclination (disclination lines) is generated in the white tone. JP-A-2005-107535 discloses a structure in which the edge portions of the pixel electrodes have a linear shape, and are inclined with respect to the pixel electrodes at an angle of 9 to 12°, whereby a restoration force of the liquid crystal and a deflecting force of the liquid crystal are increased in the edge portions of the pixel electrodes.
Since many of the edge portions in which the disclination may occur are arranged at the end of light transmitting areas, the disclination in the light transmitting areas may be avoided by the provision of the black matrix or the like. However, this results in lowering of the aperture ratio of pixel.
The disclination generated when the pixel electrodes and the common electrodes are superimposed in the EPS system is addressed in JP-A-2003-280017, and the disclination between the black matrix on the upper substrate and the edge portions of the pixel electrodes on the lower substrate is addressed in JP-A-2005-107535. In both of them, the upper electrode layer and the lower electrode layer are arranged on the identical substrate with the intermediary of the insulating layer, and the disclination when the electrode layer is formed with openings for allowing the passage of the electric field to the upper electrode layer is not addressed.